Hydrocolloids are organic polymers containing numerous hydrophilic groups such as --OH, --COOH, --SO.sub.4, --PO.sub.4 and --NH.sub.2. They may be vegetable gums such as tragacanth or animal protein such as gelatin. They are capable of uniting with water, and of dissolving or swelling in the presence of water. They have a tendency to develop adhesiveness in the presence of a sufficient amount of water.
Hydrocolloids can be dispersed in water to form a mucilage or glue. The mucilage, however, does not attach to moist surfaces well and it is not possible to form a uniform thin coating of hydrocolloid on the moist surface.